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It is the wind that defines the strength of a hurricane. The storm is not a hurricane at all until the wind reaches 74 miles per hour. Hurricane Isaac's sustained winds were not much beyond that, so it was a Category 1 storm, not two, three, four or five. But if the winds define a hurricane, it's the water that can do the most damage.

GREENE: And this morning, search and rescue operations are underway in Louisiana. Serious flooding has been reported along the shores of Lake Pontchartrain, underscoring the threat of this storm's impact as it moves inland.

Now, south of New Orleans, hundreds of homes are underwater in Plaquemines Parish. And that's where we've caught up with NPR's Greg Allen.

And, Greg, where exactly are you, and what is the situation, as we speak?

GREG ALLEN, BYLINE: Well, David, I'm in the town of Braithwaite, which is right here in Plaquemines Parish on Highway 29. That's where they close these massive floodgates, which held the water. The storm surges came over the - some of the outlying levees. The storm surge came over and started flooding that area. They closed the floodgates, and the water piled up there, 20 feet high in some areas.

Unfortunately for many people in Braithwaite, they lived on the wrong side of the floodgates. Their homes were totally inundated. That was a mandatory evacuation zone. People were told to get out. Some didn't, saying it has never flooded before. And, of course, it did flood dramatically yesterday. So we had rescue crews going in all day, bringing people out of their attics and their rooftops. They've rescued some 60 people from this area and brought them out.

Now the question is: What happens to all that water that's piled up there? I've been up looking at - on top of the levee looking down over the floodgates today, and you can see it has dropped significantly. But the president of Plaquemines Parish, Billy Nungesser, is concerned it's not going to drop quickly enough. He's been talking to folks at the state. They've agreed that they should go ahead and dig a hole through the back levee, where the water was over top, so the water can drain out quicker.

It's not exactly clear when that could happen. That could happen as early as today. It might take a little longer, depending on the weather. So that will at least get the water out from this area.

GREENE: Well, Greg, let me make sure I have a clear picture of what you're talking about. I mean, this sounds pretty dramatic. They were closing floodgates to try and, I suppose, channel water away from more populated areas, or away from heading to New Orleans. But that actually caused bigger problems for people in this rural parish who decided to stay at home.

ALLEN: Well, exactly. I mean, people in Plaquemines Parish are no strangers to flooding. I mean, it flooded in Betsy. It flooded in Katrina, dramatically. It's flooded many times in the past. And then, of course, after Katrina, the federal government and the state of Louisiana and others spent $14 billion building this storm protection system around the city, this huge ring, like, almost like a medieval fortress.

And when you look at these floodgates, it is very impressive. I mean, they've done their job admirably well. New Orleans came through this storm just with very little problems. I mean, there's been some street flooding. That's about it. So the levees have held well. The pumping has done well. So New Orleans has been great.

The only problem is for people who live in these rural areas. And as I say, they're no strangers to flooding. So I think some of them had to know this was coming, and they were told to evacuate.

GREENE: And some of these rescues, I mean, you're talking about some several dozen people, it sounds like. I mean, on their roofs, I mean, being rescued by helicopter? How dramatic was it?

ALLEN: You know, it's very reminiscent - as you know, yesterday was the anniversary of Katrina, and it was so reminiscent of Katrina because very many people we saw come in were bringing with them their pets. You know, that's one of the reasons many people stay, is they don't know what to do with pets. And, of course, the authorities have set up shelters and that kind of thing. But that was one of things that I noted yesterday, you know. Pets keep people back at home.

GREENE: I guess that is one reason to stay home, even when things get bad. Well, let me ask you about Lake Pontchartrain. We're hearing reports of flooding this morning there. How bad are things getting in that area?

ALLEN: I don't think we've seen the worst of it yet. The storm surge, of course, was a real problem for these southern parishes, the ones right on the Gulf of Mexico. But the storm surge also was dramatic in Lake Pontchartrain, so that north shore communities, the communities right along the north shore of Lake Pontchartrain, started to feel flooding yesterday. We had a levee breach in some areas up there. And then this morning, many other subdivisions were flooded.

So as far as I could tell, there are several flood divisions flooded. There are search and rescue operations, as you noted, underway there today, because those are areas that people I don't think thought were going to flood. And this really is a result of the storm surge. We have this other issue of the rainfall, too, and what it's going to do to the rivers.

GREENE: And, Greg, just let's be clear. I mean, we have the memories of Katrina still today. This storm - I mean, you covered Katrina, you're covering this one - not even close to as bad, even though there are some people who are suffering.

ALLEN: No. It's very interesting, because everybody - I think the one problem here is that people knew this was only going to be Category 1, and there was a sense that it wouldn't be as damaging as Katrina. There was many warnings from Mayor Landrieu, the major of New Orleans, from the - Governor Jindal in Louisiana and others, that we have to take this seriously, because Isaac is just such a large Category 1 storm when he came ashore. I mean, it took over 36 hours for it to pass through this city. And for 36 hours, we got 70-mile-per-hour winds. We had inches and inches of rain. And then the storm surge, because it was so large, was much greater than many people had been prepared for.

And so the massive size of it made it much more damaging than people might have thought a Category 1 hurricane should be.

GREENE: All right. NPR's Greg Allen, talking to us from Plaquemines Parish, outside New Orleans. Greg, thanks a lot.